It is known in the art relating to robotic and automatic GMAW manufacturing applications that the welding torch contact tip, being a consumable item, is usually replaced between shifts, during maintenance, or when an operator observes unacceptable welding defects on manufacturing workpieces. If the contact tip needs to be replaced during a shift, the entire welding line or cell must be shut down, which results in manufacturing delays and increased costs for the plant.
Judging when to replace a contact tip is a subjective process. Certain operators may pass (accept) some workpieces having welding defects, while other operators may have less tolerance for defects, thereby consuming more contact tips and causing more shutdowns over time. Also, usually when an operator determines that a contact tip needs to be replaced, welding defects have begun to occur, thus meaning that some workpieces may have to be repaired or scraped for quality reasons.
Historically, there have been two major contact tip failure mechanisms that have been observed over the past 50 years in constant voltage (CV) welding applications. The first is excessive wear at the front of the bore in the contact tip, which is known as a “keyhole.” The second failure mechanism is a sudden stoppage of the feeding of electrode welding wire due to a clog, jam, or excessive feeding force inside the contact tip, causing the welding arc to burn back to the contact tip, which is known as “burn back.”
The application of new GMAW welding processes, especially pulse welding, over the past decade has introduced a new contact tip failure mechanism. The high frequency and high peak current of pulse welding make the working conditions of the contact tip more critical than in CV welding. For example, it is common for a 0.9 mm (0.035 inch) solid electrode wire to be welded at 400 Amp peak current in a pulse welding application, while in constant voltage applications the typical welding current is only about 170 to 220 Amps. As the current doubles, the energy or power transferred across the contact tip to electrode wire interface quadruples. Also, in pulse welding, the welding current changes from the background current (100 Amps) to the peak current (400 Amps) in 0.15 to 0.30 ms, which corresponds to a rate of 1 to 2 million Amps per second. The high welding current and drastic current ramp causes significant arc erosion on the contact tip, thus deteriorating the contact tip. Pulse welding applications are typically set at high welding speeds, and therefore require accurate delivery of the welding current waveforms to the arc. Thus, consistent performance of the contact tip is more critical than in CV applications. The useful lifespan of a contact tip in pulse applications is significantly shorter than in CV applications. It is more likely that a contact tip will have to be replaced during a shift, and knowing when to replace the contact tip is important for preventing welding defects and minimizing operating costs.
Conventional control systems in robotic and automatic welding applications compare the commanded (desired) welding current and voltage with actual measured data. When the difference between the commanded values and the measured data is higher than a certain predetermined threshold, the system will regard the welding as out of compliance and will request immediate action such as shutdown of the manufacturing line. However, a monitoring method does not exist that assesses the useful life of a contact tip in GMAW-pulse applications, in order to output an alarm or other signal prior to the occurrence of welding defects so that the contact tip may be changed prior to failure. Further, known monitoring methods fail to adequately take into account noise factors such as arc start, arc stop, and manufacturing errors such as the dimension variation of the parts. Thus, there is a need for monitoring and assessing contact tip deterioration in real time in GMAW-pulse applications so that contact tips may be replaced prior to the occurrence of welding defects or at least in a manageable manner such that welding defects and contact tip replacement are kept at a minimum.
In the prior art, Yukimitsu (U.S. Pat. No. 1,293,088) used the average current during the arcing period to indicate the wear of the contact tip. The method was based on short-circuit welding (also known as “short-arc” GMAW), which is one of the metal transfer modes in constant voltage (CV) welding. Kitagawa (JP 2000-24779) counted the periodic variations of the welding current to interpret the deterioration of the contact tip. These periodic variations were caused by the laps or cast of the electrode wire, and the periodic changes of the contacting force and position of the welding wire inside the contact tip. Both Yukimitsu and Kitagawa were based on constant voltage (CV) welding, and thus did not consider the new failure mechanism of the contact tip in pulse applications as well as the versatility of the pulse welding control.